The present invention relates to a method for producing pulp molded articles.
Drying methods adoptable in the production of pulp molded articles include a method in which a pulp molded precursor obtained by a papermaking step is dried in a drying furnace by a predetermined heating means. This method is disadvantageous in that the resulting molded articles have poor dimensional accuracy because of incapability of shape control and that a large drying furnace is required because of the low heat conduction efficiency in drying. Additionally, the resulting molded articles have unsatisfactory surface properties and poor strength due to a low pulp density.
Another drying method is a hot pressing method in which a pulp molded precursor obtained by a papermaking step is hot-pressed in a heating mold as described in JP-A-7-42100. According to this drying method, molded articles can be obtained with good dimensional accuracy, and the drying time is shortened owing to the high heat conduction efficiency. However, because drying is accompanied by a physical phenomenon that energy necessary for evaporation is supplied by heat conduction, the time for drying, however it is shortened, is still long as compared with the time for a papermaking and dewatering step. Accordingly, a larger number of heating molds than forming molds should be prepared, which increases the cost of equipment, leading to an increase of production cost. Elevation of the heating mold temperature to reduce the drying time is apt to cause color change (sometimes scorching) of the molded articles.
Conventional containers comprising pulp molded articles are required to have surface strength to some extent so that labels adhered may not separate or a coating applied may not peel. On the other hand, containers comprising pulp molded articles are required to be flexible similarly to plastic containers.
Accordingly, an object of the present invention is to provide a method of producing a pulp molded article, which is successful in shortening the time for drying and reducing the size of drying equipment, thereby reducing the production cost.
Another object of the present invention is to provide a method of producing a pulp molded article, which is capable of controlling density and flexibility while maintaining constant surface strength thereby imparting rigidity or flexibility as required for the use of the molded article.
The present invention accomplishes the above objects by providing a method for producing a pulp molded article comprising the steps of forming a pulp molded precursor from a pulp slurry by means of a forming mold and then heat-drying the precursor, wherein the step of heat-drying includes:
a first step in which the precursor is set in a cavity of a heating mold, the cavity having a configuration in conformity to the contour of the pulp molded article to be produced, and is hot-pressed, and
a second step in which the hot-pressed precursor is taken out from the heating mold and is heat-dried in a drying furnace.
The present invention also provides an apparatus for producing a pulp molded article which can preferably be used to carry out the above-described method of producing a pulp molded article, which includes:
a first heat drying means comprising a heating mold having a cavity, a pressing member which presses a pulp molded precursor set in the cavity of the heating mold toward the cavity, and a measuring instrument for measuring the water content, the weight or the temperature of the precursor,
a second heat drying means comprising a drying furnace where the precursor which has been heat-dried to a prescribed water content by the first heat drying means is further heat-dried, and
a transfer means for transferring the precursor from the first heat drying means to the second heat drying means based on the data of the water content, the weight or the temperature of the precursor as measured with the measuring instrument.